In the manufacture of tissue products, such as bath tissue, uncreped throughdried products are now well known in the art and are commercially popular. A significant advantage of the uncreped throughdried process is the ability to make tissue sheets having high bulk and softness. The bulk of these sheets is largely due to the three-dimensional topography of the throughdrying fabrics used to produce them. This three-dimensional topography is molded into the tissue sheet during throughdrying and is tenaciously resilient, even under very high loads, due to the hydrogen bonding created during drying. While this property is very desirable in many respects, it does make subsequent modification of the sheet during the converting stage very difficult. The converting stage is generally understood to mean that portion of the total manufacturing process that occurs after the tissue sheet is formed and first rolled up into a parent roll. During converting, the sheet can be calendered and/or embossed, slit, rewound into smaller rolls and packaged for sale as bath tissue, paper towels and the like. The difficulty in modifying the sheet during converting arises particularly with respect to embossing, which typically does not readily provide permanent changes to the uncreped throughdried sheet because of its memory.
However, because of different consumer demands in various market segments, it is desirable to be able to alter the sheet properties during the converting stage of the manufacturing process. Therefore there is a need for a converting method which desirably alters the properties of the uncreped throughdried tissue sheet to produce unique tissue products.
It has now been discovered that desirable and permanent changes to the uncreped throughdried tissue basesheet can be made using a unique embossing process preceded by appropriate calendering, the combination of which essentially increases the visual and structural homogeneity of the basesheet. The resulting product possesses a unique structure and combination of properties. The embossing process includes embossing element geometry and special relationships that have been discovered to be effective in modifying the uncreped throughdried sheet topography.
Hence in one aspect, the invention resides in a method of mechanically manipulating an uncreped throughdried tissue sheet having bulky ripples oriented in the machine direction of the sheet, said method comprising:
(a) calendering the uncreped throughdried tissue sheet between a steel roll and a resilient backing roll; and
(b) embossing the calendered sheet between engraved steel embossing rolls, each of said embossing rolls containing a plurality of male embossing elements having a base and a peak which are connected by inclined sidewalls, wherein the projected area of the element base is from about 0.03 to about 0.5 square millimeters, the surface area of the element peak is from about 0.02 to about 0.3 square millimeter, the height of the element is from about 0.5 to about 3 millimeters, the minimum element-to-element spacing is from about 0.3 to about 3 millimeters, the element pattern density is from about 15 to about 70 elements per square centimeter of embossing roll surface, wherein during operation the embossing rolls are positioned relative to each other such that element bases of one roll partially overlap element bases of the other roll and engage each other at a level of from about 25 to about 60 percent engagement, whereby the tissue sheet is pinched between portions of engaging elements such that it is strained in both the machine direction and the cross-machine direction of the sheet.
In another aspect, the invention resides in an embossed uncreped throughdried tissue sheet having a base structure characterized at least in part by a stylus contact profilometry xe2x80x9cStxe2x80x9d parameter (hereinafter defined) of about 1100 microns or less, more specifically about 1000 microns or less, still more specifically about 900 microns or less, still more specifically from about 700 to about 1100 microns, and still more specifically from about 700 to about 900 microns, and/or a stylus contact profilometry xe2x80x9cStrxe2x80x9d parameter (hereinafter defined) of about 0.300 or greater, more specifically from about 0.300 to about 0.700, still more specifically from about 0.300 to about 0.600, and still more specifically from about 0.300 to about 0.500.
The impact of the method of this invention on the St parameter of a sheet, which is a z-directional measure, will depend upon the basis weight, thickness and topography of the starting material. For paper towels, which tend to be heavier and thicker than bath tissues, for example, the St parameter will likely decrease as a result of the method of this invention. On the other hand, for bath tissues, which have a lighter and thinner starting material, the St parameter will likely increase. However, for any starting material, the Str parameter, which is a measure of the visual homogeneity of the surface of the sheet, will always increase as a result of the method of this invention. These structural changes to the topography of the sheet also result in a unique combination of other properties.
Hence, in another aspect, the invention resides in a roll of a tissue sheet, wherein said tissue sheet is an uncreped throughdried sheet having a stylus contact profilometry Str parameter of from about 0.300 or greater, a stylus contact profilometry St parameter of from about 1100 microns or less, a Void Volume of about 8 or more grams per gram and a Sheet Bulk of about 12 cubic centimeters or greater per gram, said roll having a Roll Bulk of about 13 cubic centimeters or greater per gram.
In another aspect, the invention resides in a stack of tissue sheets, wherein said sheets include uncreped throughdried sheets having a stylus contact profilometry Str parameter of from about 0.300 or greater, a stylus contact profilometry St parameter of from about 1100 microns or less, a Void Volume of about 8 or more grams per gram and a Sheet Bulk of about 12 cubic centimeters or greater per gram, said stack of sheets having a Stack Bulk of about 0.25 cubic centimeters or greater per gram.
These and other aspects of the invention will be described in greater detail below.